Add backend support for sending custom acknowledgement messages

[JeffersonBledsoe]

See collective/volto-form-block#57 for more details

[JeffersonBledsoe]

@cekk ready for review

[JeffersonBledsoe]

@cekk Happy to merge this?

[JeffersonBledsoe]

@cekk I've made all of the previously noted changes.

Unfortunately the frontend for this isn't merged or released yet. Would it be possible to have this PR merged so the functionality can be tested in the frontend, and then an additional PR made later before the next release to add documentation for the required frontend version for this feature?

[coveralls]

Pull Request Test Coverage Report for Build 7450074589

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.7%) to 89.538%

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Totals
Change from base Build 7446482008:	-2.7%
Covered Lines:	659
Relevant Lines:	736

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💛 - Coveralls